This invention relates to the growth of crystals under conditions of high temperature and high pressure.
The synthesis of crystals at high temperature and high pressures, particularly diamond and cubic boron nitride, is very well established commercially. There are two principle methods employed, both form solution, namely a temperature gradient method and an allotropic change method. In the temperature gradient method, the driving force for crystal growth is the supersaturation due to the difference in solubilities of the source material and the growing crystal as the result of a temperature difference between the two. In the allotropic change method, the driving force for crystal growth is the supersaturation due to the difference in solubilities of the source material and the growing crystal as the result of an allotropic (or polymorphic) difference between the two.
The present invention provides a mass of diamond crystals, which have a size of less than 100 microns and in which mass the majority of the crystals, and preferably at least 80 percent of the mass, are macroscopically faceted single crystals. Some of the crystals may be twinned.
A mass of diamond crystals, which are predominantly macroscopically faceted single crystals may be produced by a method which includes the steps of providing a source of diamond crystals and which are substantially free of macroscopically faceted surfaces, producing a reaction mass by bringing the source diamond crystals into contact with a suitable solvent catalyst, subjecting the reaction mass to conditions of elevated temperature and pressure suitable for crystal growth in the reaction zone of a high temperature high pressure apparatus, removing the reaction mass from the reaction zone and recovering the crystals from the reaction mass, the conditions of crystal growth being chosen such that the source diamond crystals are converted to diamond crystals having developed macroscopic facets of low Miller index. The mass of crystals will generally contain at least 80% of macroscopically faceted single crystals.
The method of generating the supersaturation driving force necessary for crystal growth used in the practice of this invention depends, at least in part, and preferably predominantly, upon the difference in surface free energy between low Miller index surfaces and higher Miller index surfaces, hereinafter referred to as xe2x80x9cthe Wulff effectxe2x80x9d; higher Miller index surfaces having a higher surface free energy than lower Miller index surfaces. The equilibrium shape of a crystal occurs when the minimum total surface free energy per unit volume of crystal is attained, i.e. when the crystal is bounded by surfaces of low Miller index. Higher Miller index surfaces can be considered to comprise a series of stepped low Miller index surfaces in close proximity to one another. Such a situation is included in the term xe2x80x9chigher Miller index surfacexe2x80x9d. When a crystal is in its equilibrium shape, there exists a point whose perpendicular distance from every face is proportional to the surface free energy of that face. This is the basis of Wulff""s theorem.
It has been found that in the case of diamond, the difference in surface-free energy between high Miller index surfaces and low Miller index surfaces is large and can generate a supersaturation which sustains crystallisation when diamond crystals in various sires, including those tens of microns in size, are used. Thus, the invention has particular application to the growth of diamond crystals wherein supersaturation is created, at least in part, and preferably predominantly, by a difference in solubility of crystal surfaces of high Miller index and crystal surfaces of lower Miller index, e.g. by the reduction of surface-free energy by the substantial elimination of steps, kinks and other structural defects which characterise macroscopic high Miller index surfaces.
It has further been observed that the Wulff effect is dependent on the conditions which prevail in the reaction mass. For example, for a given solvent/catalyst and pressure applied, the Wulff effect is dependent on temperature and time, as can be seen from the graphs shown in FIGS. 1 and 2. The graph of FIG. 1 shows the temperature dependence of the Wulff effect on diamond in an iron-nickel solvent/catalyst at about 5.4 GPa, with this condition being maintained for one hour. The graph of FIG. 2 shows the temperature dependency of the Wulff effect on diamond in the same iron-nickel solvent/catalyst at about 5.4 GPa with the condition being maintained for ten hours. From these graphs, it will be noted that the larger the source crystal size the higher the applied temperature to ensure that the Wulff effect dominates and the production of a crystal mass containing a high proportion of single crystal having facets of low Miller index is achieved. Similar graphs can be produced for other solvent/catalysts and applied pressures to determine under what conditions the Wulff effect dominates.
Particles with a high portion of high Miller index surfaces will yield faceted diamond crystals more readily than particles with a low proportion of high Miller index faces. Further, particles with a low proportion of high Miller index surfaces may only facet partially and/or show dissolution facets.
The conditions of elevated temperature and pressure for crystal growth will vary according to the nature of the crystal. For diamond crystals, the elevated temperature will generally be in the range 1100 to 1500xc2x0 C. and the elevated pressure generally in the range 4.5 to 7 GPa.
The diamond crystals, may be recovered from the reaction mass using methods known in the art. For example, the most practical method is simply to dissolve away the solvent/catalyst leaving the mass of crystals. If some of the crystals are loosely bound to other crystals, they can be released by light milling or other similar action.
The method described above will primarily be used to produce a mass of crystals which has a size of less than 100 microns. However, the method may also be used for producing a mass of macroscopically faceted crystals of larger size and this also forms part of the invention.
The source diamond crystals may be provided by particles of irregular shape and substantially free of macroscopically facetted surfaces. An example of suitable source crystals is the product of a crushing operation. By way of example, FIG. 4 shows a photograph at 260xc3x97 magnification of angular source diamond crystals. The source particles may also be provided by particles which have been treated so that macroscopic facets are damaged or destroyed, and/or surfaces of high Miller index are created, and higher surface energy faces formed thereby.
The source diamond crystals may have a narrow size distribution or a relatively wide size distribution. Provided the conditions are chosen such that the Wulff effect dominates in the crystal growth, then the mass of faceted single crystals produced will have essentially the same size distribution as that of the source crystals.
Supersaturation can also be assisted by the differences in solubility between strained and less strained (or strain-free) crystals.
The mass of faceted diamond crystals has application in polishing, lapping, and grinding, allows for easier and more accurate size separation and determination, and has better flow characteristics in dry powder and slurry forms. The faceted diamond crystals also have application in the manufacture of polycrystalline products.